A variety of types of sensors have been used in the detection of chemical and/or biological processes. One type is a chemically-sensitive field effect transistor (chemFET). A chemFET includes a gate, a source, a drain separated by a channel region, and a sensitive area, such as a surface on the gate adapted for contact with a fluid, coupled to the channel region. The operation of the chemFET is based on the modulation of channel conductance caused by changes, such as changes in voltage, at the sensitive area which can be due to a chemical and/or biological reaction occurring in the fluid, for example. The modulation of the channel conductance can be sensed to detect and/or determine characteristics of the chemical and/or biological reaction that cause changes at the sensitive area. One way to measure the channel conductance is to apply appropriate bias voltages to the source and drain, and measure a resulting current flowing through the chemFET. A method of measuring channel conductance can include driving a known current through the chemFET and measuring a resulting voltage at the source or drain.
An ion-sensitive field effect transistor (ISFET) is a type of chemFET that includes an ion-sensitive layer at the sensitive area. The presence of ions in a fluid containing an analyte alters the surface potential at the interface between the ion-sensitive layer and the analyte fluid which can be due to the protonation or deprotonation of surface charge groups caused by the ions present in the fluid (i.e. an analyte solution). The change in surface potential at the sensitive area of the ISFET affects the gate voltage of the device, and thereby channel conductance, which change can be measured to indicate the presence and/or concentration of ions within the solution. Arrays of ISFETs can be used for monitoring chemical and/or biological reactions, such as DNA sequencing reactions based on the detection of ions present, generated, or used during the reactions. (See, for example, U.S. Pat. No. 7,948,015 to Rothberg et al., which is incorporated by reference herein in its entirety.) More generally, large arrays of chemFETs or other types of sensors and detectors can be employed to detect and measure static and/or dynamic amounts or concentrations of a variety of analytes in a variety of processes. For example, the processes can be chemical and/or biological reactions, cell or tissue cultures or monitoring neural activity, nucleic acid sequencing, etc.